1. Field of the Invention
The present invention relates to an imaging system and an imaging optical system for use in the imaging system.
2. Description of the Related Art
As an imaging system which images all directions at one time, an imaging system having a plurality of wide-angle lenses is known. Unlike an imaging lens for normal plane photographing, a wide-angle lens for use in such an imaging system often includes different magnification between the center thereof and the edge portion thereof.
As such, it is necessary to increase the imaging magnification in the edge portion of the lens so as to be higher than that in the center of the lens in order to compensate image quality deterioration in the edge portion of the lens in which the imaging performance thereof is likely to be deteriorated, compared to that in the center portion of the lens in the case of a constitution of a small imaging system having an equal image quality over all directions by a known wide-angle lens. In addition, in the imaging system, all directions are referred to as omnidirectional.
It is known that a lens system having a large magnification in the edge portion thereof compared to that in the center portion thereof, namely, a lens system in which the magnification per unit angle of view monotonically increases, has a short entire length. However, the thickness of the lens constituting the lens system is reduced, resulting in a deterioration in lens performance.
On the other hand, when considering the performance of a lens, the magnification in the edge portion thereof is likely to decrease compared to that in the center portion thereof. As such, the entire length of the lens system is likely to increase by increasing the imaging magnification in the edge portion compared to that in the center portion.
Japanese Patent Application Publication No. 2006-098942 describes that the magnification can be increased in an edge portion of an image by changing a projection method of a fisheye lens as a wide-angle lens to a stereoscopic projection method.